Don’t fill up your disks

I recently commented on an interesting blogg entry regarding optimizing Lightroom, where there was some confusion about why you should keep a large portion of your disk unused (and how large is large enough).

Since it became a rather long comment, and since this is an important performance factor that isn’t talked about a lot, I am simply reposting my comment as an entry here… I’m lazy like that…

I just wanted to shed some light on the "Keep space on your hard drive" tip. Michael asks if he really needs 400GB free on his 1TB drive, and the answer is yes, if maximum performance is the goal, he does. This is not so much to improve stability (once you have enough free space to write any temporary files, unfragmented, even more free space doesn’t really help you in that regard) but rather because of the way traditional hard drives work. A hard drive’s performance is not consistent across the entire surface, but varies significantly depending on which part of the drive gets read or written.

Generally, a hard drive will be fastest while writing to the outer edges of the drive’s platters, and performance will decrease when writing closer to the center of the platters. For this reason, drives fill up from the edges toward the center. The specifics of at which points and to what degree performance is cut is drive model-dependant, but generally speaking, the last parts of the disk will have about half the read and write transfer performance of the first parts.

Typical examples of this can be seen in more detailed tests of hard drives, for instance,2430-6.html (just a test of three disks I chose at random to illustrate the point).

To get the absolute maximum speed from your disk, you shouldn’t normally use much more than 20-25% of the disk, the rest should be unused space. Getting disks that are four times as big as “needed” is expensive, of course, and exactly how much you want to fill your disk, and in other words how much performance degradation you will suffer, will be an individual question. But when you’ve passed 50-60% of your disk space, this starts to be a real factor.

Finally, this relates to traditional, mechanical hard drives. The story for SSDs is far more complex. While they theoretically don’t suffer any degradation depending on what memory addresses are written, the controllers on current SSDs (especially MLC-based disks) tend to run in to a lot of optimization problems with data writes once free space starts to get scarce. So, expensive as they may be, SSDs will also give you the best performance while they are primarily empty.